Optimistic concurrency control (OCC) is an approach that aims to improve system performance by allowing multiple transactions to access shared data concurrently. OCC tends to assume that conflicts between transactions are relatively rare and, therefore, it’s more efficient to let transactions execute without first acquiring locks. But what if conflicts occur?
One way to implement OCC in Azure Cosmos DB is by leveraging entity tags (ETags). ETags are HTTP headers used to prevent conflicting updates to resources, a crucial aspect of concurrency control. It’s a mechanism that allows clients to make conditional requests to ensure that the state of the manipulated objects haven’t been changed by another client since the last time they were retrieved. In the context of Azure Cosmos DB, this provides a means for working with documents without running into race conditions or conflicting writes.
How Do ETags Work?
Every time a document in Azure Cosmos DB is read, an ETag value specific to that version of the document is retrieved. If the document is updated and saved back to the database, the ETag value previously retrieved can be passed back to ensure that the document has not been altered since it was last read. If the ETag value provided in the update request matches the current ETag value for that record in the database, the update is allowed. If not, the request fails.
Here’s a basic example:
csharp
// Fetch document
DocumentResponse productResponse = await client.ReadDocumentAsync(selfLink);
// Get current ETag
var etag = productResponse.Headers.ETag;
// Make some changes to the document, then try to update it while providing the original Etag
var ac = new AccessCondition
{
Type = AccessConditionType.IfMatch,
Condition = etag
};
await client.ReplaceDocumentAsync(selfLink, updatedProduct, new RequestOptions { AccessCondition = ac });
In this example, the call to `ReplaceDocumentAsync` would fail if the document had been updated by another client after the original read (meaning that the etag value in the if-match would not match the current state).
Conflict Resolution
When a conflict occurs (i.e., there’s a mismatch in the ETag values), the update fails and an exception is thrown. The application needs to have a mechanism for handling these exceptions and resolving the conflict effectively. Typically, this would entail re-reading the document, applying the changes to the refreshed document, and reattempting the update.
Comparison with Other Approaches
While OCC (with ETags) offers important benefits, there are alternative strategies to handling concurrency. One is pessimistic concurrency control, which assumes that conflicts are likely. Therefore, whenever a transaction begins, all the necessary locks are acquired.
Here’s a comparison of the two approaches:
| Optimistic Concurrency Control (OCC) | Pessimistic Concurrency Control (PCC) | |
|---|---|---|
| Assumptions | Conflicts are rare | Conflicts are common |
| Resource Locking | Not acquired initially | Acquired at the beginning |
| Overhead | Low (occurs only when a conflict happens) | High (since locks are always applied) |
| Use case | Useful in scenarios where write conflicts are rare | Useful where write conflicts are frequent |
In conclusion, implementing optimistic concurrency control using ETags in Azure Cosmos DB enhances the efficiency of the applications while maintaining the integrity of the data. The use of ETags allows for light-weight concurrency control without the need for explicit locking mechanisms, thereby optimizing resource usage and transaction throughput. Furthermore, it adheres to HTTP specifications and conforms to the Representational State Transfer (REST) architectural style, making it a preferred method for cloud-native applications.
Practice Test
True or False: Optimistic concurrency control can significantly reduce the chances of conflicts in a multi-user environment.
- True
Answer: True
Explanation: Optimistic concurrency control assumes that multiple transactions can complete without affecting each other, and conflict detection is only done when changes are attempted.
In optimistic concurrency control using ETags, a write operation could fail if the ETag value is different from the current ETag value of the document.
- True
Answer: True
Explanation: Write operations will only succeed if the ETag value matches, ensuring that the document has not been modified by another client since it was last read.
ETag is a mechanism to:
- A. Store the hash of the content of an HTTP resource
- B. Indicate the version of the resource
- C. Reduce the traffic in the server
- D. Control the concurrency of operations
Answer: All of the above
Explanation: ETag, or entity tag, is a mechanism in HTTP used for web caches and conditional requests from browsers for resources which indicate the version of the resource and helps in controlling concurrent operations.
True or False: Azure Cosmos DB supports Optimistic Concurrency Control using timestamps only and not ETags.
- False
Answer: False
Explanation: Azure Cosmos DB supports Optimistic Concurrency Control using both timestamps and ETags.
When a document is read in Azure Cosmos DB, it returns:
- A. Current Partition Key
- B. Current ETag Value
- C. Document data
- D. All the above
Answer: D. All the above
Explanation: When a document is read, Azure Cosmos DB returns the document’s data along with the current ETag value and Partition Key.
True or False: ETags used in optimistic concurrency control are not case-sensitive.
- True
Answer: True
Explanation: ETags are not case-sensitive as they are merely opaque tags created by the server.
The If-Match HTTP header sends the:
- A. Original ETag
- B. Modified ETag
- C. Current ETag
- D. None of the above
Answer: C. Current ETag
Explanation: The If-Match HTTP header sends the current ETag which is compared with the ETag of the resource to determine if the resource has changed.
True or False: Implementing optimistic concurrency control using ETags can result in increased latency.
- False
Answer: False
Explanation: The use of ETags can actually help to reduce latency because it enables the system to effectively handle resources without conflicts.
A successful write operation in Azure Cosmos DB returns:
- A. Updated ETAG
- B. Server-side request statistics
- C. Both A and B
- D. None of the above
Answer: C. Both A and B
Explanation: A successful write operation in Azure Cosmos DB returns both updated ETag and server-side request statistics.
True or False: ETags can be manually modified by the client-side.
- False
Answer: False
Explanation: ETags are server-defined strings that clients cannot modify. They are used to determine the equivalence of two versions of a resource.
Which among the following is not a recommended practice while implementing optimistic concurrency control using ETags in Cosmos DB:
- A. Use ETags with every SQL API
- B. Use If-Match header with current ETag value for conditional requests
- C. Manually modify the ETag
Answer: C. Manually modify the ETag
Explanation: ETags are server-generated strings for managing concurrency control and cannot be manually modified by the client.
True or False: ETags are returned in the response headers of HTTP GET requests.
- True
Answer: True
Explanation: ETags are returned in the response headers of HTTP GET requests, used to determine if a resource has changed since the last time it was requested.
True or False: Implementing optimistic concurrency control using ETags requires schema changes in your database.
- False
Answer: False
Explanation: Implementing optimistic concurrency using ETags does not require changes to the schema of your database. They are based on HTTP headers and the read and write operations of your application.
ETags are generated by Azure Cosmos DB using which attribute?
- A. _etag
- B. _ts
- C. partitionKey
- D. id
Answer: A. _etag
Explanation: Azure Cosmos DB automatically generates ETags for each resource and stores it in the _etag attribute.
True or False: The ETag protocol is specific to Azure Cosmos DB and not part of HTTP web standards.
- False
Answer: False
Explanation: ETags are a standard component of the HTTP protocol, and are used by Azure Cosmos DB to implement optimistic concurrency control.
Interview Questions
What is optimistic concurrency control in Microsoft Azure Cosmos DB?
Optimistic concurrency control in Azure Cosmos DB allows multiple users to concurrently modify a document, where the last write operation wins. This is achieved using an entity tag (ETag) property, a special system property that gets included in the response to every document read request.
How is the ETag property used in implementing optimistic concurrency control?
The ETag property is used as a version tag. When executing an update or delete operation, the value of the ETag property is passed along with the request, specifying that the operation should only succeed if the current ETag value matches the value provided.
What happens when the ETag value does not match during an operation?
If the ETag value provided in the request does not match the current ETag of the document, the operation will fail. This indicates that the document was updated by someone else in between your document read and write operations.
Is optimistic concurrency control enabled by default in Azure Cosmos DB?
No, optimistic concurrency control is not enabled by default. It must be set specifically in your requests by including the “If-Match” header with the ETag value.
What is the key benefit of using optimistic concurrency control with ETags?
The main advantage is that it improves performance and response times because it does not lock resources. It allows for multiple users to modify a document simultaneously and also brings about consistency by validating the ETag value before executing operations.
Can the ETag value be manually modified?
No, the ETag value is generated by the system and cannot be manually modified.
What is the main difference between pessimistic and optimistic concurrency control?
Pessimistic concurrency control locks the document for a single user, preventing others from modifying it until the first user finishes. Optimistic concurrency, on the other hand, allows multiple users to modify a document simultaneously based on the version of the document they originally retrieved.
Which method is best for applications where collisions are likely to occur, optimistic or pessimistic concurrency control?
Pessimistic concurrency control is suitable for situations where collisions are frequent since it locks the document for each user, preventing concurrent modifications.
What HTTP status code is returned if the ETag value does not match while performing an operation?
A 412 Precondition Failed HTTP status code is returned when the ETag value provided in the ‘If-Match’ header does not match the current ETag value.
How can you ensure that delete operations only occur on the latest version of a document in Azure Cosmos DB?
By using the ETag in the “If-Match” header during the request, you can ensure that delete operations only happen on the most recent version of a document. This is because the operation will only succeed if the ETag provided matches the current ETag of the document.
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